The determination of peptide sequences is important in the understanding of molecular basis of disease, evolutionary relationships among proteins, and the relationship between protein structure and function. The rate-limiting step in determining protein sequences is the specific cleavage of peptides and proteins into well-defined fragments. There are few chemical methods that accomplish this objective. A new method of cleaving peptides as glutamine (Gln) residues is proposed in this work. The essence of this method is the conversion of Gln residues into 2,4-diaminobutyric acid (DABA) residues using a new reagent, I,I'-bis(trifluoroacetoxy)iodobenzene. It is proposed that the resulting peptides can then be cleaved at DABA residues in one or more of three ways: direct nucleophilic cleavage, transpeptidation-Edman degradation, and glyoxal-promoted cleavage.